The Milky Way and its neighbouring galaxies are shown in this map based on Gaia satellite data: brighter regions indicate denser concentrations of stars. Credit: ESA, Gaia, DPAC

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The European Space Agency (ESA) has released the largest, most detailed map of the Milky Way, pinpointing the 3D positions of 1.1 billion stars, 400 million of which were previously unknown to science.

ESA’s Gaia space observatory mapped out the catalogue. It is expected to transform what astronomers know about the Galaxy—allowing researchers to discover new extrasolar planets, examine the distribution of dark matter, and fine-tune models of how stars evolve.

Hundreds of astronomers began to access the database as soon as it was made publicly available on September 14, says Gaia project scientist Timo Prusti, who works at ESA's European Space Research and Technology Centre in Noordwijk, the Netherlands. “My advice to the astronomical community is: please enjoy with us,” he said at a press conference in Madrid.

Gaia has already found more stars than researchers expected, which suggests that the Milky Way may be slightly bigger than previously estimated, says Gisella Clementini, a Gaia researcher at the Bologna Astronomical Observatory in Italy.

But few new results were announced at the catalogue’s unveiling, as Gaia’s team were only allowed to do limited analyses before the data release—contrary to the norm for space observatories, where mission scientists often have up to a year’s exclusive use of their data before sharing them with the world.

One notable result, however, is a new measurement of the distance of the Pleiades, a cluster of stars in the constellation Taurus that has been the subject of a long-running controversy. Where numerous measurements put the Pleiades cluster at a distance of about 135 parsecs (440 light years) from the Sun, Gaia’s predecessor, the Hipparcos mission, found it to be about 15 parsecs closer.

Gaia measured 134 parsecs, give or take 6 parsecs—suggesting that the Hipparcos findings were inaccurate. Anthony Brown, an astronomer at the Leiden Observatory in the Netherlands who chairs Gaia’s data-processing collaboration, stresses that the results are still preliminary and that they could change once Gaia collects more data. (Ultimately, Gaia should be able to measure the distances of individual stars in the cluster for the first time, rather than an average.)

But there’s scant possibility that Gaia’s results will be corrected so much that they agree with the Hipparcos results, thinks David Soderblom, an astronomer at the Space Telescope Science Institute in Baltimore, Maryland. “It’s not impossible but it sure isn’t very likely at this point,” he says. “That to me is basically the answer.” Soderblom expects that the trouble with the Hipparcos measurement may have been in corrections made to account for the unusual brightness of stars in the cluster.

Gaia launched in late 2013 and started its scientific mission in July 2014. The preliminary catalogue released today is based on its first 14 months of data-taking. Gaia does not take still exposures in the way of ordinary telescope cameras. Instead, it constantly spins on its axis every six hours, watching stars leave streaks along its 1-gigapixel detector.

By comparing scans of the sky taken six months apart, researchers are able to triangulate and measure stars' distances, using a method known as parallax that dates back to ancient Greece. For more than two million stars, the catalogue also includes accurate measurement of the stars’ distances from the Sun and their motion, obtained by comparing Gaia data with Hipparcos’s. In future releases, the catalogue will grow to include the distances and velocities of more than a billion stars.

With more years of observation, Gaia’s measurements will become so accurate that the distances of many of the galaxy’s stars will be pinpointed to within 1%.

“What Gaia is going to do is going to be phenomenal,” says Wendy Freedman, an astronomer at the University of Chicago in Illinois. “It will be the fundamental go-to place for astronomers for decades to come.”

This article is reproduced with permission and was first published on September 14, 2016.

ABOUT THE AUTHOR(S)

Davide Castelvecchi

Davide Castelvecchi is a senior reporter at Nature in London covering physics, astronomy, mathematics and computer science.

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